Last year, for the Fourth of July, Aaron Parsley and his family stayed at their river house on the Guadalupe — a one-story cabin on stilts about 50 yards from the Texas river. Parsley’s parents had bought the home because they wanted their grandkids to grow up spending time there. Parsley, a senior editor for Texas Monthly, described his niece Rosemary as a “rambunctious, expressive, willful little girl with blond hair and blue eyes,” who knows Spanish and calls him Tío. And there was his 20-month-old nephew, Clay — “a towhead who’d just learned to say the words ‘boo’ and ‘yellow.’” Clay adored his big sister, loved trailing behind her. They ate pizza. Parsley and his husband, Patrick, played hide-and-seek with their niece and nephew before bed. Later that night, around 3 a.m., Parsley awoke to the sound of thunder and rain. Outside, the river was as high as the house’s deck, about 20 feet above the ground.…